How do you measure an envelope?

Envelopes are measured with the opening on the top. Measure the height and width. A number 10 envelope measures 4-1/8” X 9-1/2”. Envelope size is ALWAYS described as height X width with the opening at the top. All window measurements are made with the opening at top, pointing to 12’oclock position on a clock. Measure the height along the vertical dimension. Measure the width along the horizontal dimension or bottom fold of the envelope.